How do I choose between different types of thermocouples?
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Choosing the right type of thermocouple is a mater of matching the thermocouple to your measurement requirement. Here are some areas to take into consideration:
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Temperature Range: The different thermocouple types have different temperature ranges. For example, Type T with its Copper leg has a max temperature of 370C or 700F. Type K on the other hand can be used up to 1260C or 2300F.
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Conductor Size: The diameter of the thermocouple wires also needs to be taken into consideration when long duration measurements are needed. For example, Type T thermocouples are rated to 370C/700F, however if your thermocouple has #14AWG wires (.064” Diameter) they are rated for 370C/700F. If your thermocouple has #30AWG wires, that drops to 150C/300F. More information can be found here (See the table on the bottom of page H-7).
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Accuracy: Type T thermocouples have the tightest accuracy of all the base metal thermocouples at ±1C or ±0.75% whichever is greater. This is followed by Type E (±1.7C or 0.5%) and Types J, K and N (±2.2C or 0.75%) for standard limits of error (per ANSI/ASTM E230).
Other important considerations are the sheath materials (if immersion probe style), insulation material (if wire or surface sensor) and sensor geometry.
Difference in Thermocouple Types:
Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. Although thermocouple calibration dictates the temperature range, the maximum range is also limited by the diameter of the thermocouple wire. That is, a very thin thermocouple may not reach the full temperature range.
Type B Thermocouples
Type B thermocouples can be used up to 1600°C with short term excursions up to 1800°C. They have a low electrical output, therefore are rarely used below 600°C. In fact the output is virtually negligible up to 50°C, therefore cold junction compensation is not usually required with this type.
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Type E Thermocouples
Type E thermocouples are often referred to as Chromel-Constantan thermocouples. They are regarded as more stable than Type K, therefore often used where a higher degree of accuracy is required.
Note - Constantan is Copper-Nickel.
Type J Thermocouples
Type J thermocouples degrade rapidly in oxidising atmospheres above 550°C. Their maximum continuous operating temperature is around 750°C though they can with stand short duration excursions to 1000°C. They are generally not used below ambient temperature due to condensation forming on the wires leading to rusting of the iron.
Note - Constantan is Copper-Nickel.
Type K Thermocouples
Type K thermocouples are the most widely used thermocouples in the Oil & Gas, and refining industries due to their wide range and low cost. They are occasionally referred to as Chromel-Alumel thermocouples. Note that above about 750°C oxidation leads to drift and the need for recalibration
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Type N Thermocouples
Type N thermocouples can handle higher temperatures than type K, and offer better repeatability in the 300 to 500°C range. They offer many advantages over Type R & S at a tenth of the cost, therefore prove to be popular alternatives.
Type R Thermocouples
Type R thermocouples cover similar applications as Type S but offer improved stability and a marginal increase in range. Consequently, Type R tend to be used in preference to Type S.
Type S Thermocouples
Type S thermocouples can be continually used at temperatures up to 1450°C. They can with stand short duration excursions up to 1650°C. They need protection from high temperature atmospheres to prevent metallic vapour ingress to the tip resulting in reduction of emf generated. Protection commonly offered is a high purity recrystallised alumina sheath. For most industrial applications thermocouples are housed in a thermowell.
Type T Thermocouples
Type T thermocouples are rarely used in industrial applications, and lend themselves more to use in laboratory situation
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FOR MORE INFORMATION ON :-
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1) Thermcouple temperature Ranges and International Color Codes
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How do I know which junction type to choose?
Sheathed thermocouple probes are available with one of three junction types: grounded, ungrounded or exposed. Check also our video about thermocouple junctions:
Grounded Junction Thermocouples
At the tip of a grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction.
This means that grounded thermocouples will have faster response times than ungrounded thermocouples.
The grounded junction is recommended for the measurement of static or flowing corrosive gas and liquid temperatures and for high pressure applications. The junction of a grounded thermocouple is welded to the protective sheath giving faster response than the ungrounded junction type.However, grounded thermocouples are highly susceptible to noise induced by ground loops, resulting in less accurate readings.
Ungrounded Junction Thermocouples
In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is slower than the grounded style.
On the other hand, the junction is electrically isolated from the sheath, which prevents electrical noise from interfering with the signal. This yields much greater temperature measurement accuracy, especially with very low-level signals.
An ungrounded junction is recommended for measurements in corrosive environments where it is desirable to have the thermocouple electronically isolated from and shielded by the sheath. The welded wire thermocouple is physically insulated from the thermocouple sheath by MgO powder (soft).
Exposed Junction Thermocouples
The thermocouple in the exposed junction style protrudes out of the tip of the sheath and is exposed to the surrounding environment. This type offers the best response time, but is limited in use to noncorrosive and nonpressurized applications.
An exposed junction is recommended for the measurement of static or flowing non-corrosive gas temperatures where fast response time is required. The junction extends beyond the protective metallic sheath to give accurate fast response. The sheath insulation is sealed where the junction extends to prevent penetration of moisture or gas which could cause errors.
Thermocouple Material selection
All TEMPDURA MgO insulated thermocouples are made using the highest purity MgO for temperatures up to 2300°F (1260°C). The thermo-elements are all ANSI special limits of error to give your measurements the best possible results. The various sheath materials are dependent on the application and the following will help you make the best selection.
304 SS Maximum temperature of 1650° (900°) and is the most widely used low temperature sheath material. It offers good corrosion resistance but is subject to carbide precipitation in the 900°F to 1600°F (480 to 870°) range.
310 SS Maximum temperature of 2100°F (1150°C) and offers good mechanical and corrosion resistance similar to 304SS. Very good heat resistance. Not as ductile as 304SS.
316 SS Maximum temperature of 1650°F (900°C) and has the best corrosion resistance of the austenitic stainless steels. Subject to carbide precipitation in the 900°F to 1600°F (480 to 870°C)
Inconel ® Maximum temperature 2150°F (1175°C) and is the most widely used thermocouple sheath material. Good high temperature strength, corrosion resistance and is resistant to chloride-ion stress corrosion, cracking and oxidation. do not use in sulfur bearing environments.
Hastelloy X Maximum temperature 2200°F (1205°) widely used in aerospace applications. Resistant to oxidizing, reducing and neutral atmospheric conditions. Excellent high temperature strength.
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Nicrobell Maximum temperature 2340°F (1300°)Highly stable in vacuum and oxidizing atmospheres. Corrosion resistance generally superior to stainless steels. Can be used in Sulfurous atmospheres at reduced temperatures. High operating.
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Refractory Oxide recrystallized, e.g.Alumina Impervious Maximum temperature 3150°F (1750°) Good choice for rare metal thermocouples.
Good resistance to chemical attack. Mechanically strong but severe thermal shock should be avoided.
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Silicon Carbide (Porous) Maximum temperature 2700°F (1500°)Good level of protection even in severe conditions. Good resistance
to reasonable levels of thermal shock. Mechanically strong when thick wall is specified but becomes brittle when aged. Unsuitable for
oxidizing atmospheres but resists fluxes.
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Impervious Mullite Maximum temperature 2880°F (1600°) Good choice for rare metal thermocouples under severe conditions.
Resists Sulfurous and carbonaceous atmospheres. Good resistance to thermal shock should be avoided.
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Choosing between a Thermocouple and RTD Sensor
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Thermocouples comprise a thermoelement which is a junction of two specifield, dissimilar alloys and a suitable two wire extension lead. The junction is a short circuit only, the EMF is generated in the temperature gradient between the hot junction and the ‘cold’ or reference junction. This characteristic is reasonably stable and repeatable and allows for a family of alternative thermocouple types (e.g. J,K,T,N) to be used.
The alternative types are defined by the nature of the alloys used in the thermoelements and each type displays a different thermal EMF characteristic.
Resistance Thermometers utilize a high precision sensing resistor, usually platinum, the resistance value of which increases with temperature. The dominant standard adopted internationally is the Pt100 which has a resistance value of 100.0 Ohms at 0°C and a change of 38.50 Ohms between 0 and 100°C (the fundamental interval).
The platinum sensing resistor is highly stable and allows high accuracy temperature sensing. Resistance thermometer sensing resistors are 2 wire devices but the 2 wires will usually be extended in a 3 or 4 wire configuration according to the application, the associated instrumentation and accuracy requirements.
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Thermocouples are, generally:
• Relatively inexpensive
• More rugged
• Less accurate
• More prone to drift
• More sensitive
• Tip sensing
• Available in smaller diameters
• Available with a wider temperature range
• More versatile
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RTD’s are, generally:
• More expensive
• More accurate
• Highly stable (if used carefully)
• Capable of better resolution
• Restricted in their range of temperature
• Stem, not tip sensitive
• Rarely available in small diameters (below 3mm)
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In both cases, the choice of thermocouple or RTD must be made to match the instrumentation and to suit the application.
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